This invention relates to radio frequency antennas, and more particularly to phased array antennas.
Microwave terrestrial and satellite communications systems are rapidly being deployed to serve communications needs. In these systems, to ensure a radio communication link between a fixed station on the ground or on a satellite and a mobile station such as an automobile or airplane, antenna systems with scanning beams have been put into practical use. A scanning beam antenna is one that can change its beam direction, usually for the purpose of maintaining a radio link, e.g. to a tower or satellite, as a mobile terminal is moving and changing direction. Another application of a scanning beam antenna is in a point-to-multipoint terrestrial link where the beams of a hub antenna or remote antenna must be pointed at different locations on a dynamic basis.
Most of the beam scanning antennas in commercial use today are mechanically controlled. This has a number of disadvantages including: limited beam scanning speed as well as limited lifetime, reliability and maintainability of the mechanical components such as motors and gears.
Electronically controlled antennas are becoming more important with the need for higher speed data, voice and video communications through geosynchronous earth orbit (GEO), medium earth orbit (MEO) and low earth orbit (LEO) satellite communication systems and point-to-point and point-to-multipoint microwave terrestrial communication systems. Additionally, new applications such as automobile radar for collision avoidance can make use of antennas with electronically controlled beam directions.
Phased array antennas are well known to provide such electronically scanned beams and could be an attractive alternative to mechanically tracking antennas because they have the features of high beam scanning (tracking) speed and low physical profile. Furthermore, phased array antennas can provide multiple beams so that multiple signals of interest can be tracked simultaneously, with no antenna movement.
Array antennas work by well-understood principles. In typical embodiments, they incorporate electronic phase shifters that provide a differential delay or phase shift to adjacent radiating elements to tilt the radiated phase front and thereby produce far-field beams in different directions depending on the differential phase shifts applied to the individual elements or, in some cases, groups of elements (sub-arrays). The typical circuit topologies (circuit diagrams) of array antennas are well known to those skilled in the art.
However, the presently used phased array antennas are too expensive for most commercial applications. Their use has been generally limited to relatively small quantities of specialized and expensive systems such as military, aircraft, and space systems. Typically, phased arrays employ hundreds or thousands of radiating elements and a correspondingly high number of phase shift elements. Their cost is proportional to the number of elements and the number of active electronic devices such as amplifiers and phase shifters.
Furthermore, conventional state-of-the-art phase shifters such as MEMS, MMIC, PIN diodes, or Ferrite devices, have high insertion loss, which at high frequencies, generally requires the use of amplifiers at the array elements. Such active arrays are very expensive and their use is limited to small volume, high performance applications where cost is not the primary driver.
Tunable ferroelectric materials are the materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected or immersed. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called xe2x80x9cferroelectricxe2x80x9d because they exhibit spontaneous polarization at temperatures below the Curie temperature. Typical tunable ferroelectric materials are barium-strontium titanate (BST) or BST composites.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Materialxe2x80x9d; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-MgOxe2x80x9d; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled xe2x80x9cCeramic ferroelectric composite material-BSTO-ZrO2xe2x80x9d; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-Magnesium Based Compoundxe2x80x9d; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled xe2x80x9cMultilayered Ferroelectric Composite Waveguidesxe2x80x9d; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled xe2x80x9cThin Film Ferroelectric Composites and Method of Makingxe2x80x9d; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled xe2x80x9cMethod of Making Thin Film Compositesxe2x80x9d; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled xe2x80x9cElectronically Graded Multilayer Ferroelectric Compositesxe2x80x9d; and U.S. Pat. No. 5,635,433 to Sengupta, entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-ZnOxe2x80x9d. These patents are hereby incorporated by reference. A copending, commonly assigned United States patent application titled xe2x80x9cElectronically Tunable Ceramic Materials Including Tunable Dielectric And Metal Silicate Phasesxe2x80x9d, by Sengupta, filed Jun. 15, 2000, discloses additional tunable dielectric materials and is also incorporated by reference. The materials shown in these patents, especially BSTO-MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage. These unique properties make these materials suitable for microwave applications such as phase shifter, tunable filters, tunable resonators, and delay lines.
U.S. Pat. No. 5,617,103 discloses a ferroelectric phase shifting antenna array that utilizes ferroelectric phase shifting components. The antennas disclosed in that patent utilize a structure in which a ferroelectric phase shifter is integrated on a single substrate with plural patch antennas. The structure disclosed in this patent may not permit the close spacing between radiating elements required in antennas that operate with high frequencies. Additional examples of phased array antennas that employ electronic phase shifters can be found in U.S. Pat. Nos. 5,079,557; 5,218,358; 5,557,286; 5,589,845; 5,617,103; 5,917,455; and 5,940,030.
One known type of phase shifter is the microstrip line phase shifter. Examples of microstrip line phase shifters utilizing tunable dielectric materials are shown in U.S. Pat. Nos. 5,212,463; 5,451,567 and 5,479,139. These patents disclose microstrip lines loaded with a voltage tunable ferroelectric material to change the velocity of propagation of the guided electromagnetic wave.
While generally the beam scanning antennas must be able to move their beams in arbitrary directions over some angular range known as the field of view or field of regard, there are many applications where the simplification of a one-dimensional beam movement or scanning is desirable.
What is needed for competitive satellite and/or terrestrial systems, whether for satellite communications, commercial radar applications (such as for cars), or for terrestrial communications applications is a phased array antenna that has the features of electronic beam scanning yet is relatively inexpensive. An array that can achieve electronic beam scanning by using low cost, low insertion loss phase shifters would fulfill a need that cannot be satisfied with conventional phase shifters. Such an antenna is the subject of this invention.
This invention provides a phased array antenna comprising an input, a feed network electronically coupled to the input, a plurality of radiating elements, a plurality of continuously voltage tunable phase shifters for receiving signals from the feed network and providing phase shifts for the signals prior to transmitting the signals to the radiating elements, and a controller for controlling the phase shift provided by the phase shifters.
This invention encompasses phased array antennas which produce beams that can be scanned in one dimension (one-dimensional) or two dimensions (two-dimensional) by using continuously adjustable phase shifters that are based on low cost, low loss voltage-tunable dielectric materials. Such antennas have many applications including, but not limited to, microwave terrestrial wireless communications, radar, and satellite communications.